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Glad you found the problem. However, and this is just my opinion, though more than a few here may share the same .02, but cheaping out on head gaskets isn't a good place to cheap out. Time will tell if you've merely reinstalled the problem. What brand of head gasket failed? I've put together 3 different 427 LS motors, and none have ever failed. I used Cometic for the first one, as Darton sleeved 427, and GM LS7 gaskets on the other 2. Are you using bolts or studs? If bolts, I'm hoping you didn't try reusing the original bolts. Don't take offense-I've seen a few posts in some forums stating the OP did in fact reuse the non-reuseable bolts. While not common, it does happen here and there.
I "think" it was the problem. We will see!
The gaskets are 3 layer MLS and look the same as the BTRs, michigan motorsports, OEM LS1 5.7 GM gaskets... They might not be made in Germany or mexico but I agree they could be of "lesser" quality. That is the reason I copper coated them just to be safe. I have three 5.3LS motors, one uses stock GM LS1 gaskets from GM performance (same 3 layer) with no coating and ARP head bolts. This engine has seen 16psi and I drive it all the time over the last 10 years. The other 5.3 has the same cheap Chinese head gaskets and ran fine with no coating/spray but uses speedmaster head studs. This motor used the same head gasket no coating and speedmaster head studs and took 2 years before it started leaking. This is why I copper coated them. My thoughts are that most of the gasket manufactures are sourced from the same place and just relabeled (BTR, Michigan motorsports etc) I'm sure felpro, GM and cometic are unique in sourcing and manufacturing but I've seen some issues with the felpros not having the correct hold orientation and the Cometics are SUPER picky about prep and flatness. I couldnt see paying a premium for the same 3 layer gaskets or even a 5 or 7 layer LS9. This is a low boost 5.3 which wouldn't have even sweat with stock 150k gaskets... no need for LS9 or special gaskets. Their is nothing special about MLS head gaskets (just layers of steel shims) and if you can reuse a stock set with just copper spray I have high confidence that this will solve my problem.
Not sure if you saw/read here but I used these gaskets (installed on the engine but never run) and then had to pull the heads due to a rust issue and then reinstalled without any copper coat. These gaskets were essentially used and then reused so I can't blame the gaskets honestly. I should have replaced them or sprayed them but I've never sprayed head gaskets before. So that is on me but new gaskets with spray I'm very confident that this should be fixed.
I'm not running bolts, Speedmaster head studs which were torqued to 65ftlbs which is within the working limits of the studs... people who go over 70ftlbs push them too hard to be reused.
Last edited by customblackbird; Oct 14, 2025 at 10:34 AM.
Something else I have noticed about my LS7 in a 2000 C5; if there's even a slight leak anywhere in my cooling system, the coolant temp actually increases, then drops way farther than usual!! IDK why, but it does. Cooling system stats:
A DeWitts radiator w/A4 connections, a 25% UD balancer, 160° stat, and an Edelbrock 8896 mechanical H2O pump. A few years back, I was ready to pull both heads, as it would lose enough coolant over the summer to go from the full mark to just below the lower level mark. No drips under the car. Freeway temps would go from 180° to 174°, then back again!! Always ran 178°-180° on freeway regardless of ambient. As I was looking at what to remove first to pull the heads, I saw a drip from a heater hose. It immediately evaporated. Then another drip in 15-20 seconds. The engine had heat sunk, increasing the pressure slightly until it dropped enough to stop. The cause? The factory sprung hose clamps had lost a tiny bit of tension. This happened several years ago, at least 4 years back. Installed a traditional hose clamp, and it has never leaked again. Saved me from buying $160 worth of head gaskets, and a Helluva lot of work!!! FWIW.....
Thats a good catch, the level prob wasn't low enough to affect your cooling performance. The expansion tanks on the new cars are constantly bleeding air from the system... as long as there is fluid in the expansion tank your likely cooling just fine. I never use the spring tension stock clamps... they stuck for the reason you explained. I don't even run worm clamps anymore... I get the Tbolt coolant/intercooler clamps and go to town. I also don't run rubber hoses and use silicone reinforced for this reason too plus they basically never wear out. This is precisely why I checked for combustion gases in the coolant... I wanted to make sure I needed to yank the heads.
Last edited by customblackbird; Oct 14, 2025 at 10:35 AM.
Motor is back together and running. I did a test for combustion Gases and it came back negative after 5-10min of being on the expansion tank. Coolant doesn’t look to have any oil in it as well in the expansion tank. I’m gona mark this as fixed for now.
i also ended up getting Jays Tstat block off and installed it. I would say it warms up a bit slower just idling. I’m hoping to take the car out today for a quick trip and confirm the engine temps and that I don’t get anything in the expansion tank like oil froth.
@Prostock John How is the warm up time? For me, I start the car and have the ECM take over closed loop by 140*F, that usually takes 10 min or so of idling just to get to that 140 mark, I don't like to drive off in open loop as I basically get into boost within 1/4 mile from my house (highway onramp). I would love to not have to worry about the Tstat sticking or failing and I'm not worried about having heat at all as its a summer/warm weather car and I have no heating system. as a side benefit air bleeding and no need to remove the stat to flush is a plus lol
Hmmm in the summer the car will warm up pretty quick. Now that it's chillier in Chicago, it would typically take about a mile of city driving to get into the 170s.
Fun fact I've driven the car when it was 40F out and I saw once saw 178F cruising lol. In this current 55F air my fan never comes on.
Pooter, I had only one previous turbo combo, had a looped hose. It was 21 years ago. I ran stock f-body fans which are pretty robust. Turbo cooling/cruising knowledge has gotten better over the years, my tuner Jon Capizzi has told me on a few occasions that turbo combos burn lazy so the move is to increase cruise and idle timing a noticeable amount over an NA combo.
My looped hose old NA and Nitrous builds ran fine cooling wise, the turbo changes things.
Update, took a short drive to take my 4yrold to his baseball game. On the way temps stayed around 175-180*F. Could of sworn I saw alittle smoke but given that the engine was all apart and I have new header wrap on the downpipe I assumed some new smells and smoke as it breaks in. After the game 1hr I drove it home and I saw temps hit maybe 185*F and come down to the mid/high 170s. Got it home and pooped the hood to inspect. I saw a little coolant at the top expansion feed from the radiator top. I just tightened that up and popped the cap. Looks exactly how I left it! Coolant looks good and clean. I did notice some copper specs/flakes probably from the head gasket spray. I want to say I hit alittle boost on the way home but it’s a slow drive and pretty short.
Been driving the car a lot this past week, prob 4 days and 160 miles. No issues with cooling or anything. Decided to pop the cap on the remote fill tank and got some crud on the bottom of the cap. I took a look down in the tank and fluid was slightly low and had a film in the tank but not a whole lot floating in the fluid. Fluid did not look as "clear" and more murky. I pulled the blank cap on the radiator and took a look which had the same film but not stuff floating in the coolant. I did put the coolant gas tester on it and ran the car up to operating temp for a bit, fluid color lightened a little but never really went yellow. I have to hold the rubber cap on the overflow and I will get a small bubble to push the tester fluid maybe once every 3-5s.
Overflow cap, looks not as "oily" like it was previously in this thread. This is looking in the overflow tank (not the radiator) Radiator tester after 10 min or so idling and light throttle blips.
